The present invention relates to a semiconductor device including a programmable read only memory (PROM) and, more particularly, to a semiconductor device including a PROM fuse element.
A memory array in a metal oxide semiconductor large scale integrated circuit (MOS LSI) memory device has spare rows and spare columns. If a row or column has a failure, then the defective row or column can be replaced by a spare row or column. For this replacement, PROM elements of fuse type (referred to as fuse elements) are provided in the memory device, and the replacement is made by melting or shortcircuiting the fuse element.
There are several known methods for programming the fuse element including: a current melting method for melting away the fuse by directing excessive current through it, a laser cutting method for cutting the fuse element by laser beams, a current shortcircuiting method for shrotcircuiting the fuse by directing current into the fuse, and a laser shortcircuiting method for shortcircuiting the fuse by laser beams. As shown in FIG. 1, a fuse element 1 can be formed on a field insulation film 3 layered on the semiconductor substrate 2 of an n-type device. A MOS LSI with such a fuse element 1 has the following disadvantages, however. The fuse element 1 is frequently programmed by melting away, cutting off or shortcircuiting the fuse element. At the time of the programming operation, heat is generated near the fuse element 1. Electrons e or holes h, which are part of paired carriers generated in the semiconductor substrate by the heat, are diffused along the substrate 2 so as to disturb the operation of an MOS LSI formed in the substrate near the fuse element 1. Particularly, in the current melting method or the current shortcircuiting method, when an MOS type drive circuit is used for programming the fuse element, the programming is inaccurately performed. To avoid this problem, the MOS type drive circuit must be sufficiently spaced from the fuse element 1.
Thus, fuse element 1, provided in the prior art, hinders improvement in integration density of the MOS LSI.
Another disadvantage with the prior art fuse element is that, due to the heat generated at the time of melting or shortcircuiting the fuse element 1, a large number of contaminant ions are generated on or in the field insulation film near the fuse element 1. The contaminant ions are, for example, alkali ions containing sodium ions and potassium ions or metal ions containing copper ions. Such contaminant ions move on or in the field insulation film 3 to form stray MOS transistors between the sources and drains of the MOS transistors formed in the substrate 2, and between the diffusion interconnection layers. The stray MOS transistors significantly deteriorate the reliability of the MOS LSI. To prevent this, the MOS LSI must be formed sufficiently separated from the fuse element. This also hinders improvements in integration density of the MOS LSI. To date, there has not been proposed a semiconductor device having means for solving the above-mentioned problems to a satisfactory degree.